Back-side deep trench isolation structure for image sensor

ABSTRACT

The present disclosure relates to an image sensor having a photodiode surrounded by a back-side deep trench isolation (BDTI) structure, and an associated method of formation. In some embodiments, a plurality of pixel regions is disposed within an image sensing die and respectively comprises a photodiode configured to convert radiation into an electrical signal. The photodiode comprises a photodiode doping column with a first doping type surrounded by a photodiode doping layer with a second doping type that is different than the first doping type. A BDTI structure is disposed between adjacent pixel regions and extending from the back-side of the image sensing die to a position within the photodiode doping layer. The BDTI structure comprises a doped liner with the second doping type and a dielectric fill layer. The doped liner lines a sidewall surface of the dielectric fill layer.

REFERENCE TO RELATED APPLICATION

This Application claims the benefit of U.S. Provisional Application No.63/014,856, filed on Apr. 24, 2020, the contents of which are herebyincorporated by reference in their entirety.

BACKGROUND

Many modern day electronic devices comprise optical imaging devices(e.g., digital cameras) that use image sensors. An image sensor mayinclude an array of pixel sensors and supporting logic. The pixelsensors measure incident radiation (e.g., light) and convert to digitaldata, and the supporting logic facilitates readout of the measurements.One type of image sensor is a backside illuminated (BSI) image sensordevice. BSI image sensor devices are used for sensing a volume of lightprojected towards a back-side of a substrate (which is opposite to afront-side of the substrate where interconnect structures includingmultiple metal and dielectric layers are built thereon). BSI imagesensor devices provide a reduced destructive interference, as comparedto front-side illuminated (FSI) image sensor device.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 illustrates a cross-sectional view of some embodiments of animage sensor comprising a photodiode surrounded by a back-side deeptrench isolation (BDTI) structure with a doped liner.

FIGS. 2A-2D illustrate a series of schematic diagrams of someembodiments of a method of forming a BDTI structure with a doped linerfor an image sensor.

FIG. 3 illustrates a cross-sectional view of some other embodiments ofan image sensor comprising a photodiode isolated by a shallow isolationwell and a BDTI structure with a doped liner.

FIG. 4 illustrates a cross-sectional view of some other embodiments ofan image sensor comprising a photodiode surrounded by a BDTI structurewith a doped liner, a shallow isolation well, and a shallow trenchisolation structure.

FIG. 5 illustrates a cross-sectional view of some embodiments of anintegrated chip comprising an image sensing die and a logic die bondedtogether where the image sensing die has a photodiode surrounded by aBDTI structure with a doped liner.

FIGS. 6-20 illustrate some embodiments of cross-sectional views showinga method of forming an image sensor having a photodiode surrounded by aBDTI structure having a conformal doped layer.

FIG. 21 illustrates a flow diagram of some embodiments of a method offorming an image sensor having a photodiode surrounded by a BDTIstructure having a doped layer.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Integrated circuit (IC) technologies are constantly being improved. Suchimprovements frequently involve scaling down device geometries toachieve lower fabrication costs, higher device integration density,higher speeds, and better performance. Due to device scaling, pixelsensors of an image sensor have smaller dimensions and are closer to oneanother. An improved electrical and optical isolation betweenneighboring pixels of the image sensor is needed in order to reduceblooming and crosstalk. Dielectric trenches and implantation wells canbe fabricated as isolation structures to isolate image sensor pixels.One kind of image sensor fabrication processes includes an implantationprocess to form deep implant wells through the depth of the photodiodeas isolation walls (e.g., an implantation process known as array deepp-well implantation). However, besides fabrication complexity, theseimplantation processes involve a thick photoresist layer which reducesexposure resolution. For example, if the critical dimension is smallerthan 0.2 μm, a precise lithography process is hardly achievable with aphotoresist layer greater than 3 μm.

In view of the above, the present disclosure relates to an image sensorcomprising a back-side deep trench isolation (BDTI) structure with adoped liner, and an associated method of formation. In some embodiments,the image sensor has a plurality of pixel regions disposed within animage sensing die. The pixel regions respectively have a photodiodeconfigured to convert radiation into an electric signal. The photodiodeincludes a photodiode doping column with a first doping type surroundedby a photodiode doping layer with a second doping type that is differentthan the first doping type. A BDTI structure is disposed betweenadjacent pixel regions and extends from a back-side of the image sensingdie to a position within the photodiode doping layer. The BDTI structurecomprises a doped liner with the second doping type lining a sidewallsurface of a deep trench of the photodiode doping layer and a fillinglayer disposed in remaining inner space of the deep trench. With theBDTI structure extending deeply and functioned as the deep depletion andisolation structure between neighboring pixels, no deep implantationfrom a front-side of the sensing die is needed.

In addition, in some embodiments, a cyclic cleaning process is performedafter forming the deep trench and before forming the doped liner in thedeep trench, such that a defective upper portion of the photodiodedoping layer exposing to the deep trench and a bowing tip at top cornerof the deep trench are removed or at least reduced, leaving a smoothsidewall surface and a less bowing neck for the deep trench. As aresult, a smooth and uniform filling result can be more easily achievedduring subsequent trench filling processes. In some further embodiments,the doped liner is formed by a low temperature epitaxial processfollowed by a laser annealing process for dopant activation. Thereby,without introducing unwanted extraordinary heat budget, the doped lineris formed in conformal, smoothly, and with less defects. More details ofsome embodiments of the methods of forming the doped liner are describedbelow associated with FIGS. 2A-2D and FIGS. 13-15 of manufacturingprocess illustration.

FIG. 1 illustrates a cross-sectional view of an image sensor 100 havinga photodiode 104 surrounded by a BDTI structure 111 with a doped liner114 according to some embodiments. The image sensing die 134 has afront-side 122 and a back-side 124. The image sensor 100 comprises animage sensing die 134 having a plurality of pixel regions that may bearranged in an array comprising rows and/or columns, such as pixelregions 103 a, 103 b shown in FIG. 1 . The pixel regions 103 a, 103 brespectively comprises the photodiode 104 configured to convert incidentradiation or incident light 120 (e.g., photons) into an electric signal.In some embodiments, the photodiode 104 comprises a first region such asa photodiode doping column 104 a having a first doping type (e.g.,n-type doping by dopants such as phosphorus, arsenic, antimony, etc.)and an adjoining second region such as a photodiode doping layer 128having a second doping type (e.g., p-type doping by dopants such asboron, aluminum, indium, etc.) that is different than the first dopingtype.

The BDTI structure 111 is disposed between and isolate adjacent pixelregions 103 a, 103 b. The BDTI structure 111 may extends from theback-side 124 of the image sensing die 134 to a position within thephotodiode doping layer 128 or extend through the photodiode dopinglayer 128 as shown in FIG. 1 . In some embodiments, the BDTI structure111 comprises the doped liner 114 with the second doping type (e.g.,p-type doping) and a dielectric fill layer 112. The doped liner 114lines a sidewall surface of a deep trench of the photodiode doping layer128, and the dielectric fill layer 112 fills a remaining space of thedeep trench. The doped liner 114 may comprise doped silicon or otherdoped semiconductor material with boron or other p-type dopants. Thedielectric fill layer 112 may be made of silicon dioxide, siliconnitride, and/or other applicable dielectric material. The doped liner114 and the dielectric fill layer 112 may extend laterally along theback-side 124 of the image sensing die 134. In some embodiments, abowing tip at the top corner of the BDTI structure 111 has a bowingangle in a range of about 8° to 15° from an upper sidewall of the BDTIstructure 111 to a vertical line perpendicular to a lateral plane of thephotodiode doping layer 128. In some embodiments, the bowing tip issmaller than about 8°. As disclosed hereabove and hereafter, the bowingtip may be introduced by a manufacturing step of forming a deep trenchfor the BDTI structure 111 by an etching process. The etching processmay involve anisotropic etching processes including dry etching and wetetching that may create an under-cut profile. The bowing top may be thenremoved or at least reduced by a cyclic cleaning process, leaving asmooth sidewall surface and a less bowing neck for the deep trench.

In some embodiments, a plurality of color filters 116 are arranged overthe back-side 124 of the image sensing die 134. The plurality of colorfilters 116 are respectively configured to transmit specific wavelengthsof incident radiation or incident light 120. For example, a first colorfilter (e.g., a red color filter) may transmit light having wavelengthswithin a first range, while a second color filter may transmit lighthaving wavelengths within a second range different than the first range.In some embodiments, the plurality of color filters 116 may be arrangedwithin a grid structure overlying a plurality of the photodiodes 104.

In some embodiments, a plurality of micro-lenses 118 is arranged overthe plurality of color filters 116. Respective micro-lenses 118 arealigned laterally with the color filters 116 and overlie the pixelregions 103 a, 103 b. In some embodiments, the plurality of micro-lenses118 have a substantially flat bottom surface abutting the plurality ofcolor filters 116 and a curved upper surface. The curved upper surfaceis configured to focus the incident radiation or incident light 120(e.g., light towards the underlying pixel regions 103 a, 103 b). Duringoperation of the image sensor, the incident radiation or incident light120 is focused by the micro-lenses 118 to the underlying pixel regions103 a, 103 b. When incident radiation or incident light of sufficientenergy strikes the photodiodes 104, it generates an electron-hole pairthat produces a photocurrent. Notably, though the micro-lenses 118 isshown as fixing onto the image sensor in FIG. 1 , it is appreciated thatthe image sensor may not include micro-lens, and the micro-lens may beattached to the image sensor later in a separate manufacture activity.

FIGS. 2A-2D illustrate a series of schematic diagrams of a method ofpreparing a deep trench 1202 and forming the doped liner 114 on asidewall surface of the deep trench 1202 for an image sensor accordingto some embodiments. FIGS. 2A-2D show some intermediate parts of theimage sensors disclosed in this application such as the image sensor 100disclosed in FIG. 1 above during manufacturing processes. The deeptrench 1202 is not a straight column because of attainable formationmethod. For example, as shown in FIG. 2A, the deep trench 1202 is formedfrom the back-side 124 of the photodiode doping layer 128 by an etchingprocess. The etching process involves anisotropic etching processesincluding dry etching and wet etching such as using tetramethylammoniumhydroxide (TMAH) as one of the etchant. The deep trench 1202 may have anunder-cut profile and a bowing tip at the top corner of the deep trench1202. The bowing tip may have a bowing angle θ₁ in a range of about 15°to 30° from an upper sidewall of the deep trench 1202 to a vertical lineperpendicular to plane of the photodiode doping layer 128. Also, anupper portion of the photodiode doping layer 128 exposing to the deeptrench 1202 is damaged because of dislocation and native oxide formationand converts to a defective layer 128′ with a thickness T_(d) as adamage result of the etching process.

FIG. 2B shows the deep trench 1202 after a cyclic cleaning process. Insome embodiments, the cyclic cleaning process is used to remove thedefective layer 128′ and smoothen sidewall surfaces of the deep trench1202. The cyclic cleaning process may comprise using solutions of atleast two different etchants such as hydrofluoric acid (HF) and ammoniaand hydrogen peroxide mixtures (APM) alternatively for multiple cycles.This process is different from a general cleaning process such as a wetcleaning using hydrofluoric acid solution, a SiCoNi pre-cleaning, and/orother plasma enhanced pre-cleaning processes since the cyclic cleaningprocess intends to remove a substantial portion of the upper portion ofthe photodiode doping layer 128 to completely remove the defective layer128′ and achieve a smooth surface for subsequent deposition process. Insome embodiments, the cyclic cleaning process removes the defectivelayer 128′ with the thickness T_(d) in a range of about 1-20 nm, or atleast about 20 nm. As a result, sidewall surfaces of the deep trench1202 are smoothen, and the bowing tip is reduced. A bowing width W_(b)is defined as a lateral distance from the bowing tip to a body of thedeep trench 1202 as shown in FIG. 2B. The bowing width W_(b) may belinearly reduced as the cycles of the cleaning process increase. Theresulted bowing tip may have a bowing angle θ₂ reduced to be smallerthan 15° from an upper sidewall of the deep trench 1202 to a verticalline perpendicular to plane of the photodiode doping layer 128. Forexample, the upper portion of the photodiode doping layer 128 may beremoved for around 21 nanometers (nm) while each cycled removes around 6angstroms (Å). The bowing width W_(b) may be reduced to around 10 nmwith 36 cycles of such cleaning. As a result, a sidewall profile of theBDTI structure is formed with less bowing neck, and performance of theimage sensor can be improved because trench filling quality would beimproved with a straighter sidewall of the deep trench 1202.

Then, as shown in FIG. 2C, a doped liner precursor 114′ is formed on thesmoothen sidewall surfaces of the deep trench 1202 through an epitaxialdeposition process before filling remaining spaces of the deep trench1202. The doped liner precursor 114′ is formed by a lower temperatureepitaxial deposition process with a delta doping of p-type dopants. Insome embodiments, the doped liner precursor 114′ may have a thickness ofaround 1.3 nm with a boron concentration around 1×10¹⁹ cm⁻³. In someembodiments, a dopant concentration of the doped liner precursor 114′may be in a range between approximately 5×10¹⁹ atoms/cm³ toapproximately 2×10²⁰ atoms/cm³. A thickness of the doped liner precursor114′ may be in a range between approximately 0.5 nm and approximately 3nm. The doped liner precursor 114′ may have a thickness not exceeding 10nm. A thicker doped liner, a higher forming temperature, or a smallerconcentration of dopants adversely affects the number of white pixelsand/or the dark current of the image sensor. For example, a doped linerprecursor with a thickness of around 10 nm and the same dopantconcentration as the doped liner precursor 114′ results more than 5times of the number of white pixels and/or the dark current of the imagesensor. A doped liner with a dopant concentration smaller than 8×10¹⁹cm⁻³ greatly increases the number of white pixels and may even resultfailure of the image sensor.

As shown in FIG. 2D, a dopant activation process follows the formationof the doped liner precursor 114′ to facilitate dopants diffusion fromthe doped liner precursor 114′ to an adjoining portion of the and toform a doped liner 114. In some embodiments, the dopant activationprocess is a laser annealing process such as a dynamic surface annealprocess and may include multiple rounds to achieve uniform dopantdistribution. As an example, the dopants can be boron. A surfaceconcentration of boron can be greater than 10²⁰ cm³, and a diffusiondepth can be around 20 nm, at which depth from top the boronconcentration is reduced to around 10¹⁵ cm⁻³. In some embodiments, thebowing width W_(b) and the bowing angle θ₂ of the deep trench 1202 maysubstantially maintained after the formation of the doped liner 114 asdescribed in FIG. 2C and FIG. 2D.

FIG. 3 illustrates a cross-sectional view of an image sensor 300comprising a photodiode 104 isolated by a doped shallow isolation well110 and a BDTI structure 111 with a doped liner 114 according to someother embodiments. Features of the image sensor 100 shown in FIG. 1 andother figures can be incorporated in the image sensor 300 whenapplicable. In some embodiments, the BDTI structure 111 may have a depthD in a range of between approximately 1.5 μm and approximately 5 μm. Alateral dimension W of the BDTI structure 111 may have a range betweenapproximately 0.1 μm and approximately 0.3 μm. The lateral dimension ofthe BDTI structure 111 should be sufficient to perform the formation ofthe doped liner 114 and other layers inside the BDTI structure (forexample, as described associated with FIGS. 13-16 below). A surfaceroughness of the doped liner 114 may be smaller than 3 Å. The conformityof the doped liner 114 from top to bottom is greater than 90%. In someembodiments, the more conformal thickness, the smoother surface, and themore uniform dopant concentration of the doped liner 114 is achieved byusing the cyclic cleaning process, the epitaxial deposition process, andthe dopant activation process described above associated with FIGS.2B-2D. More details of the formation method of the doped liner 114 arealso discussed associated with FIGS. 13-15 .

In addition, in some embodiments, a doped shallow isolation well 110 isdisposed between and isolate adjacent pixel regions 103 a, 103 b,extending from the front-side 122 of the image sensing die 134 to aposition within the photodiode doping layer 128. The doped shallowisolation well 110 may have the second doping type (e.g., p-typedoping). In some embodiments, a bottom portion of the BDTI structure 111may be disposed within a recessed top surface of the doped shallowisolation well 110. In this case, the doped shallow isolation well 110may reach less than a half or even less than ¼ depth of the BDTIstructure 111. The doped shallow isolation well 110 may be verticallyaligned with the BDTI structure 111 (e.g. sharing a common center line126). The BDTI structure 111 and the doped shallow isolation well 110collectively function as isolations for the pixel regions 103 a, 103 b,such that crosstalk and blooming among the pixel regions 103 a, 103 bcan be reduced. The BDTI structure 111 and the doped shallow isolationwell 110 also collectively facilitate depletion of the photodiode 104during the operation since the BDTI structure 111 and the doped shallowisolation well 110 provide additional p-type dopants to the photodiode104, such that full well capacity is improved.

In some embodiments, the BDTI structure 111 further comprises a high-kdielectric liner 113 disposed between the doped liner 114 and thedielectric fill layer 112 and separating the doped liner 114 fromdielectric fill layer 112. The high-k dielectric liner 113 may also be aconformal layer. The high-k dielectric liner 113 may comprise aluminumoxide (Al₂O₃), hafnium oxide (HfO₂), hafnium silicon oxide (HfSiO),hafnium aluminum oxide (HfAlO), tantalum oxide (Ta₂O₅), or hafniumtantalum oxide (HMO), for example. Other applicable high-k dielectricmaterials are also within the scope of the disclosure. In someembodiments, the high-k dielectric liner 113 may have a thickness rangebetween approximately 30 nm and approximately 100 nm and may be made ofcomposite of multiple high-k dielectric materials. The doped liner 114,the high-k dielectric liner 113, and the dielectric fill layer 112 maylaterally extend along the back-side 124 of the image sensing die 134.

In some embodiments, a floating diffusion well 204 is disposed betweenthe adjacent pixel regions 103 a, 103 b from the front-side 122 of theimage sensing die 134 to a position within the photodiode doping layer128. In some embodiments, the BDTI structure 111 extends to a locationoverlying the floating diffusion well 204. The BDTI structure 111 andthe floating diffusion well 204 may be vertically aligned (e.g. sharinga common center line 302). A transfer gate 202 is arranged over thephotodiode doping layer 128 at a position laterally between thephotodiode 104 and the floating diffusion well 204. During theoperation, the transfer gate 202 controls charge transfer from thephotodiode 104 to the floating diffusion well 204. If the charge levelis sufficiently high within the floating diffusion well 204, a sourcefollower transistor (not shown) is activated and charges are selectivelyoutput according to operation of a row select transistor (not shown)used for addressing. A reset transistor (not shown) can be used to resetthe photodiode 104 between exposure periods.

FIG. 4 illustrates a cross-sectional view of an image sensor 400comprising a photodiode 104 surrounded by a BDTI structure 111 with adoped liner 114 according to some other embodiments. Features of theimage sensors 100 and 300 shown in FIG. 1 and FIG. 3 and the imagesensor shown in other figures can be incorporated in the image sensor400 when applicable. In addition, in some embodiments alternative toFIG. 3 , the doped shallow isolation well 110 may be separated from theBDTI structure 111 by the photodiode doping layer 128. Also, a shallowtrench isolation (STI) structure 402 may be disposed between theadjacent pixel regions 103a, 103b from the front-side 122 of the imagesensing die 134 to a position within the photodiode doping layer 128.The STI structure 402 and the BDTI structure 111 may be verticallyaligned (e.g. sharing a common center line 404, which may share a centerline with the doped shallow isolation well 110). In some embodiments,the doped shallow isolation well 110 extends from the front-side 122 ofthe image sensing die 134 to a position within the photodiode dopinglayer 128 and surrounds the STI structure 402. The doped shallowisolation well 110 may separate the STI structure 402 from thephotodiode doping layer 128 and/or the BDTI structure 111. In somefurther embodiments, the photodiode doping columns 104a may extend toreach on a lateral portion of the doped liner 114 of the BDTI structure111 from the back-side 124 of the image sensing die 134. The BDTIstructure 111, the doped shallow isolation well 110, and the STIstructure 402 collectively function as isolations for the pixel regions103a, 103b, such that crosstalk and blooming among the pixel regions 103a, 103 b can be reduced. The doped liner 114 of the BDTI structure 111and the doped shallow isolation well 110 also collectively facilitatedepletion of the photodiode 104 during the operation, such that fullwell capacity is improved.

FIG. 5 illustrates a cross-sectional view of an integrated chip 500comprising an image sensing die 134 and a logic die 136 bonded togetherwhere the image sensing die 134 has a photodiode 104 surrounded by aBDTI structure 111 with a doped liner 114 according to some otherembodiments. Features of the image sensors 100, 300, and 400 shown inFIG. 1 , FIG. 3 , and FIG. 4 and the image sensors shown in otherfigures can be incorporated in the image sensing die 134 whenapplicable. The image sensing die 134 may further comprise a compositegrid 506 disposed between and overlying pixel regions 103 a, 103 b. Thecomposite grid 506 may comprise a metal layer 502 and a dielectric layer504 one stacked another at the back-side 124 of the image sensing die134. A dielectric liner 508 lines sidewall and top of the composite grid506. The metal layer 502 may be or be comprised of one or more layers oftungsten, copper, aluminum copper, or titanium nitride. The metal layer502 may have a thickness range between approximately 100 nm andapproximately 500 nm. The dielectric layer 504 may be or be comprised ofone or more layers of silicon dioxide, silicon nitride, or thecombination thereof. The dielectric layer 504 may have a thickness rangebetween approximately 200 nm and approximately 800 nm. The dielectricliner 508 may may be or be comprised of an oxide, such as silicondioxide. The dielectric liner 508 may have a thickness range betweenapproximately 5 nm and approximately 50 nm. Other applicable metalmaterials are also within the scope of the disclosure. A metallizationstack 108 may be arranged on the front-side 122 of the image sensing die134. The metallization stack 108 comprises a plurality of metalinterconnect layers arranged within one or more inter-level dielectric(ILD) layer 106. The ILD layer 106 may comprise one or more of a low-kdielectric layer (i.e., a dielectric with a dielectric constant lessthan about 3.9), an ultra low-k dielectric layer, or an oxide (e.g.,silicon oxide). In some embodiments, the BDTI structure 111 may extendthrough the photodiode doping layer 128 and reach on the ILD layer 106or a gate dielectric layer of transistor devices such as a gatedielectric of the transfer gate 202.

The logic die 136 may comprise logic devices 142 disposed over a logicsubstrate 140. The logic die 136 may further comprises a metallizationstack 144 disposed within an ILD layer 146 overlying the logic devices142. The image sensing die 134 and the logic die 136 may be bonded faceto face, face to back, or back to back. As an example, FIG. 4 shows aface to face bonding structure where a pair of intermediate bondingdielectric layers 138, 148, and bonding pads 150, 152 are arrangedbetween the image sensing die 134 and the logic die 136 and respectivelybond the metallization stacks 108, 144 through a fusion or a eutecticbonding structure.

FIGS. 6-20 illustrate some embodiments of cross-sectional views 600-2000showing a method of forming an image sensor having a photodiodesurrounded by a BDTI structure with a doped liner. In some embodiments,the formation of the BDTI structure includes a cyclic cleaning processfollowing etching of deep trenches such that a defective layer isremoved and sidewall surfaces of the deep trenches are smoothed. Thenthe doped liner is formed on the smoothen sidewall surfaces of the deeptrenches through an epitaxial deposition process before fillingremaining spaces of the deep trenches. As a result, a sidewall profileof the BDTI structure is formed with less bowing neck, and performanceof the image sensor can be improved. Though doping types are providedfor varies doped regions as an example, it is appreciated that reverseddoping types can be used for these doped regions to realize a reversedimage sensor device structure.

As shown in cross-sectional view 600 of FIG. 6 , a substrate 102′ isprovided for an image sensing die 134. In various embodiments, thesubstrate 102′ may comprise any type of semiconductor body (e.g.,silicon/germanium/CMOS bulk, SiGe, SOI, etc.) such as a semiconductorwafer or one or more die on a wafer, as well as any other type ofsemiconductor and/or epitaxial layers formed thereon and/or otherwiseassociated therewith. For example, a pixel array deep p-type well 132may be formed on a handling substrate 102. The handling substrate 102can be or be comprised of a highly doped p-type substrate layer. A pixelarray deep n-type well 130 may be formed on the pixel array deep p-typewell 132. The pixel array deep n-type well 130 and the pixel array deepp-type well 132 may be formed by implantation processes. In someembodiments, a photodiode doping layer 128 is formed as an upper portionof the substrate 102′. The photodiode doping layer 128 may be formed bya p-type epitaxial process. In some embodiments, a plurality of shallowtrench isolation (STI) structures 402 is formed at a boundary and/orbetween adjacent pixel regions 103 a, 103 b from a front-side 122 of theimage sensing die 134 to a position within the photodiode doping layer128. The one or more STI structures 402 may be formed by selectivelyetching the front-side 122 of the image sensing die 134 to formshallow-trenches and subsequently forming an oxide within theshallow-trenches.

As shown in cross-sectional view 700 of FIG. 7 , dopant species areimplanted into the photodiode doping layer 128 to form doped region. Aplurality of photodiode doping columns 104 a may be formed by implantingn-type dopant species respectively within the pixel regions 103 a, 103b. A plurality of doped shallow isolation wells 110 may be formed byimplanting p-type dopant species into the photodiode doping layer 128between adjacent pixel regions 103 a, 103 b. The plurality of dopedshallow isolation wells 110 may be formed from the front-side 122 of theimage sensing die 134 to a position deeper than the STI structures 402.The doped shallow isolation wells 110 may respectively be centrallyaligned with the STI structures 402. In some embodiments, the photodiodedoping layer 128 may be selectively implanted according to patternedmasking layers (not shown) comprising photoresist.

As shown in cross-sectional view 800 of FIG. 8 , a transfer gate 202 isformed over a front-side 122 of the image sensing die 134. The transfergate 202 may be formed by depositing a gate dielectric layer and a gateelectrode layer over the substrate 102′. The gate dielectric layer andthe gate electrode layer are subsequently patterned to form a gatedielectric 802 and a gate electrode 804. In some embodiments, animplantation process is performed within the front-side 122 of the imagesensing die 134 to form a floating diffusion well 204 along one side ofthe transfer gate 202 or opposing sides of a pair of the transfer gates202.

As shown in cross-sectional view 900 of FIG. 9 , a metallization stack108 may be formed on the front-side 122 of the image sensing die 134. Insome embodiments, the metallization stack 108 may be formed by formingan ILD layer 106, which comprises one or more layers of ILD material, onthe front-side 122 of the image sensing die 134. The ILD layer 106 issubsequently etched to form via holes and/or metal trenches. The viaholes and/or metal trenches are then filled with a conductive materialto form the plurality of metal interconnect vias 510 and metal lines512. In some embodiments, the ILD layer 106 may be deposited by aphysical vapor deposition technique (e.g., PVD, CVD, etc.). Theplurality of metal interconnect layers may be formed using a depositionprocess and/or a plating process (e.g., electroplating, electro-lessplating, etc.). In various embodiments, the plurality of metalinterconnect layers may comprise tungsten, copper, or aluminum copper,for example.

As shown in cross-sectional view 1000 of FIG. 10 , the image sensing die134 can be then bonded to one or more other dies. For example, the imagesensing die 134 can be bonded to a logic die 136 prepared to have logicdevices 142. The image sensing die 134 and the logic die 136 may bebonded face to face, face to back, or back to back. For example, thebonding process may use a pair of intermediate bonding dielectric layers138, 148, and bonding pads 150, 152 to bond the metallization stacks108, 144 of the image sensing die 134 and the logic die 136. The bondingprocess may comprise a fusion or a eutectic bonding process. The bondingprocess may also comprise a hybrid bonding process including metal tometal bonding of the bonding pads 150, 152, and dielectric to dielectricbonding of the intermediate bonding dielectric layers 138, 148. Anannealing process may follow the hybrid bonding process, and may beperformed at a temperature range between about 250° C. to about 450° fora time in a range of about 0.5 hour to about 4 hours, for example.

As shown in cross-sectional view 1100 of FIG. 11 , the image sensing die134 is thinned on a back-side 124 that is opposite to the front-side122. The thinning process may partially or completely removes thehandling substrate 102 (See FIG. 10 ) and allow for radiation to passthrough the back-side 124 of the image sensing die 134 to the photodiode104. In some embodiments, the image sensing die 134 is thinned to exposethe photodiode doping columns 104 a, such that radiation can reach onthe photodiode more easily. Then a later formed BDTI structure or asemiconductor layer there in (see BDTI structure 111 or doped liner 114in FIG. 16 for example) may be formed to reach on a surface of thephotodiode doping columns 104 a. The substrate 102′ may be thinned byetching the back-side 124 of the image sensing die 134. Alternatively,the substrate 102′ may be thinned by mechanical grinding the back-side124 of the image sensing die 134. As an example, the substrate 102′ canbe firstly grinded to a thickness range between approximately 17 μm andapproximately 45 μm. Then, an aggressive wet etch can be applied tofurther thin the substrate 102′. An example of the etchant may includehydrogen fluoride/nitric/acetic acid (HNA). A chemical mechanicalprocess and a tetramethylammonium hydroxide (TMAH)) wet etching may thenfollow to further thin a thickness range between approximately 2.8 μmand approximately 7.2 μm so the radiation can pass through the back-side124 of the image sensing die 134 to reach the photodiode 104.

As shown in cross-sectional view 1200 of FIG. 12 , the substrate 102′ isselectively etched to form deep trenches 1202 within the back-side 124of the image sensing die 134 laterally separating the photodiode 104. Insome embodiments, the substrate 102′ may be etched by forming a maskinglayer onto the back-side 124 of the image sensing die 134. The substrate102′ is then exposed to an etchant in regions not covered by the maskinglayer. The etchant etches the substrate 102′ to form the deep trenches1202 extending into the substrate 102′. In some alternative embodiments,the substrate 102′ or the photodiode doping layer 128 is etchedthoroughly in depth when forming the deep trenches 1202, and the deeptrenches 1202 extend through the substrate 102′ and may reach on the ILDlayer 106, such that a complete isolation is achieved. In variousembodiments, the masking layer may comprise photoresist or a nitride(e.g., SiN) patterned using a photolithography process. The maskinglayer may also comprise atomic layer deposition (ALD) or plasma enhancedCVD oxide layer with a thickness range between about 200 angstrom (Å) toabout 1000 angstrom (Å). In various embodiments, the etchant maycomprise a dry etchant have an etching chemistry comprising a fluorinespecies (e.g., CF₄, CHF₃, C₄F₈, etc.) or a wet etchant (e.g.,hydroflouric acid (HF) or tetramethylammonium hydroxide (TMAH)). Thedeep trenches 1202 may have a depth range between approximately 1.5 μmand approximately 5 μm. A lateral dimension may have a range betweenapproximately 0.1 μm and approximately 0.3 μm. The deep trench 1202 mayhave an under-cut profile and a bowing tip at the top of the deep trench1202. Also, an upper portion of the photodiode doping layer 128 forms adefective layer 128′ exposing to the deep trench 1202 as a damage resultof the etching process and may include native oxide and other unwantedimpurity layers.

As shown in cross-sectional view 1300 of FIG. 13 , a cyclic cleaningprocess is performed to the deep trenches 1202 to remove the defectivelayer 128′ and smoothen sidewall surfaces of the deep trench 1202. Thecyclic cleaning process may comprise using solutions of hydrofluoricacid (HF) and ammonia and hydrogen peroxide mixtures (APM) alternativelyfor multiple cycles. For example, the defective layer 128′ may beremoved for around 21 nanometers (nm) while each cycled removes around 6angstrom (Å). As a result, the bowing tip is reduced beside smoothingsidewall surfaces of the deep trench 1202. The resulted bowing tip mayhave a bowing angle θ₂ smaller than 15° from an upper sidewall of thedeep trench 1202 to a vertical line perpendicular to plane of thephotodiode doping layer 128. In some embodiments, the bowing angle θ₂ issmaller than 8° such that a better filling result can be achieved. Insome embodiments, some other cleaning processes may follow the cycliccleaning process. An additional wet cleaning process using HF and aremote plasma SiCoNi cleaning may be performed to further improvecharacters of dark current and white pixels of the image sensor. Apre-cleaning process using HF solution may be used prior to the cycliccleaning process to remove native oxide. As an example, the pre-cleaningprocess may use a HF solution with a 130 (water):1 (chemical) ratio for90 seconds and a queue time less than two hours.

As shown in cross-sectional view 1400 of FIG. 14 , a doped linerprecursor 114′ is formed on sidewall and bottom surfaces of the deeptrenches 1202. In some embodiments, the doped liner precursor 114′ maybe formed by a low temperature epitaxial growth process, for example, anepitaxial growth process with a temperature lower than 500° C.Processing gases may comprise silane (SiH₄), dichlorosilane (DCS, orH₂SiCl₂), diboran (B₂H₆), hydrogen (H₂) or other applicable gases. Theepitaxial growth process may be performed in a low pressure chemicalvapor deposition epitaxial tool at a pressure in a range betweenapproximately 4 torr and approximately 200 torr at a temperature rangebetween approximately 400° C. to approximately 490° C. to form anepitaxial doped layer as the doped liner precursor 114′ with a thicknessin a range between approximately 0.5 nm and approximately 3 nm, such asaround 2 nm. The doped liner precursor 114′ may not exceed a thicknessof 10 nm, and may further not exceed 3 nm to sufficiently limit defectsand roughness. The forming temperature should not be higher than 490° C.since a higher forming temperature would cause a lower dopantconcentration and an increased roughness. The doped liner precursor 114′is formed on the smoothen sidewall surfaces of the deep trench 1202 andwould result a better conformity than conventional beamline implanttechnique, which suffers shadowing effect for three-dimensionalstructure and cannot achieve desired conformity. The doped linerprecursor 114′ is formed with a delta doping. A concentration of boroncan be in a range of from about 5×10¹⁹ cm⁻³ to about 2×10²⁰ cm⁻³, andmay further not less than 1×10¹⁹ cm⁻³. A thicker doped liner or asmaller concentration of dopants adversely affects the number of whitepixels and/or the dark current of the image sensor.

As shown in cross-sectional view 1500 of FIG. 15 , a dopant activationprocess is then performed to facilitate diffusion and to form the dopedliner 114. In some embodiments, the dopant activation process comprisesor is a laser annealing process or a dynamic surface annealing process.As an example, the annealing may use a green laser, and the annealingtemperature may be in a range between approximately 800° C. andapproximately 1100° C. for a time in a range between approximately 10nanoseconds and approximately 100 nanoseconds. The dopant activationprocess is beneficial to low thermal budget products, especiallycompared to other approaches such as a deposition process followed by athermal drive-in process, which either can't provide enough junctiondepth or not acceptable for low thermal budget product because of thehigh temperature junction drive-in and anneal for damage recovery anddopant activation.

As shown in cross-sectional view 1600 of FIG. 16 , the deep trenches1202 are then filled with dielectric materials. In some embodiments, ahigh-k dielectric liner 113 is formed within the deep trenches 1202along the doped liner 114. The high-k dielectric liner 113 may be formedby deposition techniques and may comprise aluminum oxide (AlO), hafniumoxide (HfO), tantalum oxide (TaO) or other dielectric materials having adielectric constant greater than that of silicon oxide. The doped liner114 and the high-k dielectric liner 113 line sidewalls and bottomsurfaces of the deep trenches 1202. In some embodiments, the doped liner114 and the high-k dielectric liner 113 may extend over the back-side124 of the image sensing die 134 between the deep trenches 1202. Adielectric fill layer 112 is formed to fill a remainder of the deeptrenches 1202. In some embodiments, a planarization process is performedafter forming the dielectric fill layer 112 to form a planar surfacethat extends along an upper surface of the high-k dielectric liner 113and the dielectric fill layer 112. The doped liner 114, the high-kdielectric liner 113, and the dielectric fill layer 112 may subject to aplanarization process that removes lateral portions of the overlying thedielectric fill layer 112, the high-k dielectric liner 113, and thedoped liner 114 directly overlying pixel regions 103 a, 103 b. In someembodiments, the high-k dielectric liner 113, and the dielectric filllayer 112 may be deposited using a physical vapor deposition techniqueor a chemical vapor deposition technique. As a result, the BDTIstructure 111 is formed in the substrate 102′, extending from theback-side 124 to a position within the photodiode doping layer 128. TheBDTI structure 111 is formed between and isolate adjacent pixel regions103 a, 103 b.

The cleaning process, the epitaxial growth process, and the activationprocess described above provide an improved conformal doping liner witha more conformal thickness, a more uniform doping concentration, and asmoother interface with the underlying photodiode doping layer 128. Asurface roughness can also be reduced compared to the surface roughnessof a doped liner formed without the cyclic cleaning process or theepitaxial growth process.

FIGS. 17-19 show some embodiments of a method of forming color filters116 overlying the photodiode doping columns 104 a. As shown incross-sectional view 1700 of FIG. 17 , a metal layer 502 and adielectric layer 504 are stacked over the substrate 102′ along the backside 124 of the image sensing die 134. The metal layer 502 may be or becomprised of one or more layers of tungsten, copper, aluminum copper, ortitanium nitride. Other applicable metal materials are also within thescope of the disclosure. The dielectric layer 504 may be or be comprisedof one or more layers of silicon dioxide, silicon nitride, or thecombination thereof. The dielectric layer 504 may function as a hardmask layer. As shown in cross-sectional view 1800 of FIG. 18 , an etchis performed to the metal layer 502 and the dielectric layer 504 to formthe composite grid 506. The openings 1802 may be centrally aligned withthe photodiode doping columns 104 a so that the composite grid 506 isarranged around and between the photodiode doping columns 104 a.Alternatively, the openings 1802 may be laterally shifted or offset inat least one direction from the photodiode doping columns 104 a so thatthe composite grid 506 at least partially overlies the photodiode dopingcolumns 104 a. Then, a dielectric liner 508 is formed lining sidewalland top of the composite grid 506, and lining the openings 1802. Thedielectric liner 508 may be formed using a conformal depositiontechnique, such as, for example, chemical vapor deposition (CVD) orphysical vapor deposition (PVD). The dielectric liner 508 may be, forexample, formed of an oxide, such as silicon dioxide. As shown in FIG.19 , color filters 116 corresponding to pixel sensors are formed in theopenings 1802 of the corresponding pixel sensors. The color filter layeris formed of a material that allows light of the corresponding color topass therethrough, while blocking light of other colors. Further, thecolor filters 116 may be formed with assigned colors. For example, thecolor filters 116 are alternatingly formed with assigned colors of red,green, and blue. The color filters 116 may be formed with upper surfacesaligned with that of the composite grid 506. The color filters 116 maybe laterally shifted or offset in at least one direction from thephotodiode doping columns 104 a of the corresponding pixel sensors.Depending upon the extent of the shift or offset, the color filters 116may partially fill the openings of the corresponding pixel sensors andmay partially fill the openings of pixel sensors neighboring thecorresponding pixel sensors. Alternatively, the color filters 116 may besymmetrical about vertical axes aligned with photodiode centers of thecorresponding pixel sensors. The process for forming the color filters116 may include, for each of the different colors of the colorassignments, forming a color filter layer and patterning the colorfilter layer. The color filter layer may be planarized subsequent toformation. The patterning may be performed by forming a photoresistlayer with a pattern over the color filter layer, applying an etchant tothe color filter layer according to the pattern of the photoresistlayer, and removing the pattern photoresist layer.

As illustrated by FIG. 20 , micro-lenses 118 corresponding to the pixelsensors are formed over the color filters 116 of the corresponding pixelsensors. In some embodiments, the plurality of micro-lenses may beformed by depositing a micro-lens material above the plurality of colorfilters (e.g., by a spin-on method or a deposition process). Amicro-lens template having a curved upper surface is patterned above themicro-lens material. In some embodiments, the micro-lens template maycomprise a photoresist material exposed using a distributing exposinglight dose (e.g., for a negative photoresist more light is exposed at abottom of the curvature and less light is exposed at a top of thecurvature), developed and baked to form a rounding shape. Themicro-lenses 118 are then formed by selectively etching the micro-lensmaterial according to the micro-lens template.

FIG. 21 illustrates a flow diagram of some embodiments of a method 2100of forming an image sensor having a photodiode surrounded by a BDTIstructure having a conformal doped layer.

While disclosed method 2100 is illustrated and described herein as aseries of acts or events, it will be appreciated that the illustratedordering of such acts or events are not to be interpreted in a limitingsense. For example, some acts may occur in different orders and/orconcurrently with other acts or events apart from those illustratedand/or described herein. In addition, not all illustrated acts may berequired to implement one or more aspects or embodiments of thedescription herein. Further, one or more of the acts depicted herein maybe carried out in one or more separate acts and/or phases

At act 2102, a substrate is prepared for an image sensing die. Aphotodiode and a doped isolation well are formed in the substrate from afront-side of the image sensing die. In some embodiments, an epitaxiallayer is formed over a handling substrate as a photodiode doping layer,and photodiode doping columns and/or doped isolation wells may be formedby implanting dopant species into the epitaxial layer. The dopedisolation wells may be formed by a selective implantation to form aplurality of columns extending into the photodiode doping layer. In someembodiments, a shallow trench isolation region may be formed within thefront-side of the image sensing die by selectively etching the substrateto form shallow-trenches and subsequently forming a dielectric (e.g. anoxide) within the shallow-trenches. FIGS. 6-7 illustrate cross-sectionalviews corresponding to some embodiments corresponding to act 2102.

At act 2104, a transfer gate is formed on the front-side of the theimage sensing die. A metallization stack is then formed over thetransfer gate. FIGS. 8-9 illustrate cross-sectional views correspondingto some embodiments corresponding to act 2104.

At act 2106, in some embodiments, the image sensor is bonded to one ormore other dies such as a logic die or other image sensing dies. FIG. 10illustrates a cross-sectional view corresponding to some embodimentscorresponding to act 2106.

At act 2108, the substrate is selectively etched to form deep trenchesbetween adjacent sensing pixel regions and extending into the substratefrom a back-side of the image sensing die. The deep trenches may have acenter line aligned with that of the doped isolation well and/or theshallow trench isolation region. In some embodiments, the substrate isthinned before etching to form the deep trenches. A handling substratemay be partially or completely removed from the back-side of the imagesensing die. FIGS. 11-12 illustrate cross-sectional views correspondingto some embodiments corresponding to act 2108.

At act 2110, a cyclic cleaning process is performed to the deeptrenches. FIG. 13 illustrates a cross-sectional view corresponding tosome embodiments corresponding to act 2110.

At act 2112, a doped liner is formed along sidewall and bottom of thedeep trenches. In some embodiments, the doped liner can be formed by alow temperature epitaxial process. FIG. 14 illustrates a cross-sectionalview corresponding to some embodiments corresponding to act 2112.

At act 2114, an annealing process is performed to facilitate dopantdiffusion from the doped liner to underlying photodiode doping layer.FIG. 15 illustrates a cross-sectional view corresponding to someembodiments corresponding to act 2114.

At act 2116, remaining spaces of the deep trenches are filled withdielectric materials. A high-k dielectric liner may be formed within thedeep trenches onto the doped liner. FIG. 16 illustrates across-sectional view corresponding to some embodiments corresponding toact 2116.

At act 2118, anti-reflective layer and composite grid are formed on theback side of the image sensing die. FIGS. 17-18 illustratecross-sectional views corresponding to some embodiments corresponding toact 2118.

At act 2120, color filters and micro-lenses are formed on the back-sideof the image sensing die. FIGS. 19-20 illustrate cross-sectional viewscorresponding to some embodiments corresponding to act 2120.

Therefore, the present disclosure relates to an image sensor having aphotodiode surrounded by a BDTI structure, and an associated method offormation. The BDTI structure comprises a doped liner lining a sidewallsurface of a deep trench and a dielectric layer filling a remainingspace of the deep trench. By forming the disclosed BDTI structure thatfunctions as a doped well and an isolation structure, the implantationprocesses from a front-side of the image sensing die is simplified, andthus the exposure resolution and the full well capacity of thephotodiode are improved, and the blooming and crosstalk are reduced. Byperforming a cyclic cleaning process to remove a defective layer withina deep trench of the BDTI structure and then forming a thin epitaxialdoped liner in the deep trench, a smooth interface is provided betweenthe doped liner and the underlying photodiode doping layer, and thuswhite pixels and dark current are significantly reduced. In some furtherembodiments, the BDTI structure can be used beyond image sensors, suchas a semiconductor device including a deep trench capacitor.

In some embodiments, the present disclosure relates to a method offorming an image sensor. A plurality of photodiodes is formed for aplurality of pixel regions from a front-side of an image sensing die. Aphotodiode is formed to have a photodiode doping column with a firstdoping type surrounded by a photodiode doping layer with a second dopingtype that is different than the first doping type. A deep trench isformed between adjacent pixel regions by etching the photodiode dopinglayer from a back-side of the image sensing die. An upper portion of thephotodiode doping layer exposed to the deep trench is converted to adefective layer during the etching of the deep trench. A cyclic cleaningprocess of at least two different etchants is performed alternatively toremove the defective layer. A doped liner with the second doping type isformed lining a sidewall surface of the deep trench. A dielectric filllayer is formed filling an inner space of the deep trench to form aback-side deep trench isolation (BDTI) structure.

In some alternative embodiments, the present disclosure relates tomethod of forming an image sensor. The method comprises formingphotodiodes for a plurality of pixel regions from a front-side of animage sensing die. A photodiode is formed to have a photodiode dopingcolumn with a first doping type surrounded by a photodiode doping layerwith a second doping type that is different than the first doping type.A doped isolation well is formed from the front-side of the imagesensing die by implanting dopants into the photodiode doping layerthrough a plurality of implanting processes. A gate structure and ametallization stack are formed on the front-side of the image sensingdie, wherein the metallization stack comprises a plurality of metalinterconnect layers arranged within one or more inter-level dielectriclayers. The image sensing die is bonded to to a logic die from thefront-side of the image sensing die, wherein the logic die compriseslogic devices. A deep trench is formed between adjacent pixel regions byetching from a back-side of the image sensing die. A cyclic cleaningprocess of at least two different etchants is performed alternatively toremove an upper portion of the photodiode doping layer exposed to thedeep trench. A doped liner with the second doping type is formed lininga sidewall surface of the deep trench. A dielectric fill layer is formedfilling an inner space of the deep trench to form a back-side deeptrench isolation (BDTI) structure.

In yet other embodiments, the present disclosure relates to an imagesensor. The image sensor comprises an image sensing die having afront-side and a back-side opposite to the front-side. A plurality ofpixel regions is disposed within the image sensing die and respectivelycomprises a photodiode configured to convert radiation that enters fromthe back-side of the image sensor die into an electrical signal. Thephotodiode comprises a photodiode doping column with a first doping typesurrounded by a photodiode doping layer with a second doping type thatis different than the first doping type. A BDTI structure is disposedbetween adjacent pixel regions and extending from the back-side of theimage sensor die to a position within the photodiode doping layer. TheBDTI structure comprises a doped liner with the second doping type and adielectric fill layer, the doped liner lining a sidewall surface of thedielectric fill layer.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method of forming an image sensor, comprising: forming a plurality of photodiodes for a plurality of pixel regions from a front-side of an image sensing die, wherein a photodiode is formed to have a photodiode doping column with a first doping type surrounded by a photodiode doping layer with a second doping type that is different than the first doping type; forming a deep trench between adjacent pixel regions in the photodiode doping layer from a back-side of the image sensing die, wherein an upper portion of the photodiode doping layer exposed to the deep trench is converted to a defective layer during the forming of the deep trench; performing a cyclic cleaning process of at least two different etchants alternatively to remove the defective layer; forming a doped liner precursor with the second doping type lining a sidewall surface of the deep trench, the doped liner precursor having a thickness smaller than 10 nm and a doping concentration greater than 1×10¹⁹cm−3; forming a doped liner by performing an annealing process to facilitate dopant diffusion from the doped liner precursor to an adjoining portion of the photodiode doping layer; and forming a dielectric fill layer filling an inner space of the deep trench to form a back-side deep trench isolation (BDTI) structure; wherein the doped liner is formed with a surface dopant concentration greater than 1×10²⁰cm−3 and a depth of 20 nm at which the dopant concentration is reduced to around 10¹⁵cm−3.
 2. The method of claim 1, wherein performing the cyclic cleaning process comprises using solutions of hydrofluoric acid (HF) and ammonia and hydrogen peroxide mixtures (APM) alternatively for multiple cycles.
 3. The method of claim 1, wherein the cyclic cleaning process removes at least about 1˜20 nm of the upper portion of the photodiode doping layer.
 4. The method of claim 1, wherein the doped liner precursor is formed by an epitaxial deposition process under a temperature lower than 500° C.
 5. The method of claim 1, wherein the annealing process comprises multiple rounds of a dynamic surface anneal process.
 6. The method of claim 1, wherein the cyclic cleaning process reduces a bowing angle of a bowing tip at a top corner of the deep trench to be smaller than 15° from an upper sidewall to a vertical line perpendicular to a lateral plane of the photodiode doping layer.
 7. The method of claim 4, wherein the annealing process is a laser annealing process.
 8. The method of claim 1, wherein a bowing width and a bowing angle of the deep trench are reduced by the cyclic cleaning process.
 9. The method of claim 1, wherein the BDTI structure is formed through the photodiode doping layer.
 10. The method of claim 1, wherein the doped liner is formed to reach on a surface of a doped isolation well.
 11. A method of forming an image sensor, comprising: forming photodiodes for a plurality of pixel regions from a front-side of an image sensing die, wherein a photodiode is formed to have a photodiode doping column with a first doping type surrounded by a photodiode doping layer with a second doping type that is different than the first doping type; forming a doped isolation well from the front-side of the image sensing die by implanting dopants into the photodiode doping layer through at least one implanting process; forming a gate structure and a metallization stack on the front-side of the image sensing die, wherein the metallization stack comprises a plurality of metal interconnect layers arranged within one or more inter-level dielectric layers; bonding the image sensing die to a logic die from the front-side of the image sensing die, wherein the logic die comprises logic devices; forming a deep trench between adjacent pixel regions in a back-side of the image sensing die; performing a cyclic cleaning process to remove an upper portion of the photodiode doping layer exposed to the deep trench; forming a doped liner with the second doping type lining a sidewall surface of the deep trench; and forming a dielectric fill layer filling an inner space of the deep trench to form a back-side deep trench isolation (BDTI) structure; wherein the cyclic cleaning process reduces a bowing angle of a bowing tip at a top corner of the deep trench to be smaller than 15° from an upper sidewall to a vertical line perpendicular to a lateral plane of the photodiode doping layer.
 12. The method of claim 11, wherein performing the cyclic cleaning process comprises using solutions of hydrofluoric acid (HF) and ammonia and hydrogen peroxide mixtures (APM) alternatively for multiple cycles.
 13. The method of claim 11, further comprising: forming a shallow trench isolation (STI) structure between the adjacent pixel regions from the front-side of the image sensing die to a position within the photodiode doping layer; wherein the deep trench is formed to expose the STI structure.
 14. The method of claim 11, wherein the deep trench is formed to expose the doped isolation well.
 15. A method of forming an image sensor, comprising: preparing an image sensing die having a front-side and a back-side opposite to the front-side; forming a plurality of pixel regions within the image sensing die and respectively comprising a photodiode including a photodiode doping column with a first doping type surrounded by a photodiode doping layer with a second doping type that is different than the first doping type; and forming a deep trench between adjacent pixel regions in the photodiode doping layer from the back-side of the image sensing die, wherein an upper portion of the photodiode doping layer exposed to the deep trench is converted to a defective layer during the forming of the deep trench; performing a cyclic cleaning process of at least two different etchants alternatively to remove the defective layer; forming a doped liner precursor with the second doping type lining a sidewall surface of the deep trench, the doped liner precursor having a thickness smaller than 10 nm and a doping concentration greater than 1×10¹⁹cm−3; forming a doped liner by performing an annealing process to facilitate dopant diffusion from the doped liner precursor to an adjoining portion of the photodiode doping layer; and forming a dielectric fill layer filling an inner space of the deep trench to form a back-side deep trench isolation (BDTI) structure; wherein a bowing width and a bowing angle of the deep trench are reduced by the cyclic cleaning process.
 16. The method of claim 15, wherein the doped liner and the dielectric fill layer of the BDTI structure extend laterally along the back-side of the image sensing die; and wherein a lateral portion of the doped liner is formed overlying the photodiode doping column.
 17. The method of claim 15, further comprising: forming a doped isolation well with the second doping type between the adjacent pixel regions and extending from the front-side of the image sensing die to a position within the photodiode doping layer; wherein the doped isolation well directly contacts the BDTI structure.
 18. The method of claim 15, further comprising: forming a shallow trench isolation (STI) structure between the adjacent pixel regions from the front-side of the image sensing die to a position within the photodiode doping layer.
 19. The method of claim 18, wherein the BDTI structure extends through the STI structure.
 20. The method of claim 15, wherein the bowing width is of about 10 nm, and the bowing angle is in a range of about 8° to 15° from an upper sidewall of the BDTI structure to a vertical line perpendicular to a lateral plane of the photodiode doping layer. 